1. Field of the Invention
The present invention relates to a process for producing a silicon carbide material. More particularly, the present invention relates to a process for producing a silicon carbide material especially in the form of fibers, sheets or three-dimensionally structured articles, which is useful as a reinforcing material for composite materials and as a heat-insulating material.
2. Description of the Related Art
It is known to produce silicon carbide materials by conversion of a precursor consisting of an organic silicon compound.
In the conventional precursor-converting method, as disclosed in, for example, Japanese Examined Patent Publication (Kokoku) No. 59-33,681, a polydimethyl silane is synthesized from dimethyldichlorosilane in the presence of metallic sodium by a dechlorination reaction, and then converted to a polycarbosilane by a thermal decomposition reaction. The resultant polycarbosilane is melt-spun, the resultant polycarbosilane filaments are heat-treated at a temperature of from 100.degree. C. to 190.degree. C. in air to thermally oxidize the filaments and to make the filaments non-fusible, and then the resultant oxidized filaments are sintered at a temperature of from 1200.degree. C. to 1500.degree. C. in an inert gas stream.
Japanese Unexamined Patent Publication (Kokai) No. 1-131,016 discloses a process for producing silicon carbide which is in the form of a mass consisting of extremely fine particles having a specific surface area of 100 m.sup.2 /g or more and useful as a carrier of catalysts for petrochemistry, especially of catalysts which may be heated at a high temperature of about 1000.degree. C. This process comprises a step of reacting a silicon monoxide (SiO) gas with carbon. In this process, in a first reaction region, a mixture of silicon dioxide (SiO.sub.2) with silicon (Si) is heated to a temperature of 1100.degree. C. to 1400.degree. C. under a pressure of 0.1 to 1.5 hPa, to produce silicon monoxide (SiO) gas, and then in a second reaction region, the silicon monoxide gas is brought into contact with finely divided reactive carbon having a specific surface area of 200 m.sup.2 /g or more at a temperature of 1100.degree. C. to 1400.degree. C. to convert the carbon to silicon carbide. The resultant silicon carbide is usable as a carrier of catalysts for chemical reactions. Therefore, the silicon carbide is required to have a large specific surface area and a high durability of the specific surface area.
Japanese Unexamined Patent Publication (Kokai) No. 60-231,820 discloses a process for coating a surface of a carbon fiber with silicon carbide by heat-reacting a carbon fiber with silicon monoxide (SiO) gas. This process is, however, disadvantageous in that only surface portions of the carbon fibers can be converted to silicon carbide, the inner portions of the carbon fibers substantially cannot be completely converted to silicon carbide, and thus the resultant product exhibits a poor resistance to oxidation at high temperature.
As an attempt to overcome the above-mentioned problems, Japanese Unexamined Patent Publication (Kokai) No. 6-192,917 discloses a process for producing silicon carbide fibers by reacting silicon monoxide gas with activated porous carbon fibers provided with uniform fine pores formed therein and having a size of several angstroms to several hundreds of angstroms, and having a specific surface area of 100 to 2500 m.sup.2 /g and a fiber thickness of 5 to 100 .mu.m, at a temperature of 800.degree. to 2000.degree. C. In this process, if the specific surface area is too small, silicon monoxide cannot fully penetrate into the inside of the activated porous carbon fibers, and thus it becomes impossible to produce target fibers completely consisting of silicon carbide. Also, if the specific surface area is too large, the activated porous carbon fibers per se exhibit a poor mechanical strength and thus the yield of the activated porous carbon fibers obtained from a pore-forming process is low. This is a problem of the above-mentioned process.
The activated porous carbon fibers can be produced by various conventional methods, for example, the method of Japanese Examined Patent Publication (Kokoku) No. 61-58,567, in which cellulose fibers, for example, rayon fibers are used as a starting material, the method of Japanese Unexamined Patent Publication (Kokai) No. 61-282,430 in which acrylic fibers are used as a starting material, the method of Japanese Unexamined Patent Publication (Kokai) No. 60-199,922 in which a fibrous material produced from a petroleum pitch is used as a starting material, and the method of Japanese Examined Patent Publication (Kokoku) No. 57-43,647 in which phenolic resin fibers are used as a starting material. In those prior art methods, the carbon fibers produced by dehydrate-carbonizing the starting material fibers in an inert gas atmosphere at a temperature of 200.degree. C. to 400.degree. C., are brought into contact with an oxidative gas, for example, water vapor, oxygen gas or carbon dioxide gas, while heating them at a temperature of 450.degree. C. to 1000.degree. C. higher than the dehydrate-carbonizing temperature, to activate the carbon fibers.
The silicon carbide fibers produced by the above-mentioned methods are disadvantageous in their poor tensile strength. To solve these problems, several attempts have been made by the inventors of the present invention. One attempt is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 7-18,520 a method of heat-treating the silicon carbide fibers produced by the above-mentioned methods in a gas atmosphere containing an oxidative gas at a temperature of 800.degree. C. to 2000.degree. C. Also, in another attempt of the inventors of the present invention, a sheet comprising activated porous carbon fibers having a specific surface area of 100 to 3000 m.sup.2 /g is reacted with a silicon monoxide gas at a temperature of 800.degree. to 2000.degree. C. under a reduced pressure and the resultant silicon carbide fiber sheet is heat-treated in a gas atmosphere containing an oxidative gas to enhance the mechanical strength of the silicon carbide fibers.
As mentioned above, when the reaction efficiency of the carbon material with the silicon monoxide gas is enhanced by causing the silicon monoxide gas to fully penetrate into the inside of the activated porous carbon material, it is necessary to make the thickness of the carbon material (fibers) and the size of the fine pores formed in the carbon material as small as possible and to make the specific surface area of the activated porous carbon material as large as possible within a permissible range. However, the resultant porous carbon material exhibits a very poor mechanical strength. Therefore, the fibrous or sheet-shaped silicon carbide material produced from the activated porous carbon material has a very low mechanical strength and is not satisfactory for practical use.
The mechanical (tensile) strength of the silicon carbide material can be enhanced by heat-treating the silicon carbide material in the oxidative gas-containing atmosphere. However, the resultant product has a low modulus of elasticity and thus is unsatisfactory for use as a structural material.
Although the heat-treatment method of the silicon carbide material in an oxidative gas-containing atmosphere effectively increases the materials mechanical strength, it has been confirmed by elemental analysis that a certain portion of the silicon carbide is disadvantageously converted to silicon oxides, for example SiO.sub.2, and thus the heat resistance of the silicon carbide material is reduced in response to the production of silicon oxides.
Namely, the prior art processes for producing silicon carbide fibers, sheets or three-dimensionally structural articles are not always satisfactory. Therefore, there is strong demand for a new process for producing a silicon carbide material having an enhanced mechanical strength and modulus of elasticity.